(1) Field of the Invention
The present invention relates to an air diffuser which provides air or gas to a wastewater treatment plant, a combined septic tank, an aerobic reactor such as an aerobic fermentation tank, or a fish-breeding tank, which require adequate dissolved oxygen. More particularly, the present invention relates to a multi-disc air diffuser which can supply uniform and minute air bubbles in a great quantity over a large area of an aeration tank so that the concentration of dissolved oxygen is increased, thereby making it possible to treat wastewater efficiently.
(2) Description of the Related Arts
Today, as the industrialization and the urbanization continues to increase, the environmental pollution is getting worse and the various types of wastewater, dirty water and sewage are discharged. Thus, facilities for treating them have also highly developed and costs for treating have increased.
Now in order to treat the wastewater, a biological treatment method optionally combined with a physical and chemical treatment method is usually used. The biological treatment method can be divided into an anaerobic treatment method and an aerobic treatment method.
The advantages of the anaerobic treatment method are that it does not need an oxygen supply and can obtain flammable methane gas. However, the method has disadvantages of having a long reaction time and a smell.
The aerobic treatment method has a disadvantage of needing energy for supplying oxygen. However, because its reaction time is short and organic materials can be removed completely, the aerobic treatment method makes up the majority of wastewater treatment processes.
In the aerobic treatment method, an oxygen supply is essential for the growth and activity of aerobic microorganism, and an air diffuser is used to supply oxygen.
The air diffuser is installed in the inside of an aeration tank (a reaction tank). It is important that the concentration of dissolved oxygen DO is kept adequately in the aeration tank. In the aeration tank, because gas is dispersed in wastewater of liquid state, the size and the staying amount of air bubbles, and a contacting and flowing property of gas and liquid states affect significantly in an operation condition, capability and efficiency of the aeration tank.
In order to increase contact surface and a coefficient of mass transfer between gas and liquid in the aeration tank, it is efficient to increase the staying amount of air bubbles. To achieve this, it is necessary to reduce the size of the air bubble and to generate uniformly.
This is because the air bubbles are increased and raised by combing to each other in the aeration tank, and the raising speed of the air bubbles is increased in proportion to the size of the air bubbles.
If the air bubbles have a large and non-uniform size, the staying time of the air bubbles in the aeration tank is shorter and the standard oxygen transfer efficiency SOTE is dropped.
Therefore, an air diffuser having high SOTE is required. That is, if an air diffuser can generate uniform air bubbles with decreasing their size, it is possible to reduce the number of the air diffuser installed in the aeration tank, thereby reducing the installation charge of treatment facilities.
As described above, as the industrialization and the urbanization continues to increase, various types of wastewater, dirty water and sewage are discharged, and facilities for treating them have also highly developed and costs for treating have increased.
Now in wastewater treatment processes, in order to keep the dissolved oxygen DO in the aeration tank at an adequate concentration, an electric energy cost is highly required. If an air diffuser having high SOTE is developed to generate uniform air bubbles with decreasing their size, the number and the time of running an air blower can be reduced than a conventional air diffuser, thereby decreasing considerably the electric energy cost.
If the electric energy cost is reduced, it can contribute to reduce carbon dioxide gas that causes the greenhouse effect.
The capability of an air diffuser can be degraded by a great deal of organic or inorganic matter included in wastewater. That is, by depositing sludge in the outside of the air diffuser, the generation of the air bubbles may be disturbed. Further, because the wastewater can be flowed backward into the air diffuser by the water pressure of the aeration tank, it becomes possible to shorten the lifetime of apparatus such as an air blower.
If an air diffuser for solving these problems is provided, it is possible to reduce replacement and maintenance costs.
From this point of view, conventional air diffusers used today will be looked at here. Conventional air diffusers are divided into a pipe type, a ball type and a disc type.
The disc type of air diffuser has some problems that air holes may be clogged by depositing of sludge and the capacity of an air blower should be large enough to keep the dissolved oxygen at a constant concentration, which is caused by a pressure load. Further, the air holes can be enlarged by using repeatedly and the air bubbles are expanded, so that the staying time of the air bubbles is shorten and the SOTE is dropped.
Further, in an upper disc and a lower disc being adjacent to each other, the lower surface of the upper disc may be adhered to cover completely the upper surface of the lower disc. In this case, there is a problem that the air bubbles held in an air room cannot be generated uniformly and minutely by the lower surface of the upper disc.
As a result, there is a problem that the size of the air bubbles may be non-uniform or enlarged by growing the air bubbles in the lower surface of the upper disc.
Further, there is a problem that the air bubbles are not supplied steadily by a pressure load due to introduce and deposit sludge in the air nozzle. In this case, in order to remove the sludge, the multi-disc air diffuser should be disassembled. Accordingly, the maintenance of the air diffuser becomes difficult.